Devices for measuring peak signal levels are commonly used in hearing aid amplifiers, which require some measure of automatic gain control (AGC). In such applications, the magnitude of amplified acoustic signals is constantly monitored, and amplifier gain varied to maintain signal levels within the operating range of the amplifier. The devices of the present invention were specifically developed for use in hearing aid amplifiers; however, the devices can be used in many practical applications calling for the generation of a signal indicative of the peak magnitude of an alternating voltage signal.
In a preferred embodiment of the detector described more fully below, the general design object was to provide a peak magnitude detector that could be manufactured as part of an integrated circuit hearing aid amplifier (except for components such as capacitors), and that could be operated from a supply voltage in the order of one volt. In the past, such hearing aid amplifiers have employed peak level detectors (or half-wave rectifiers performing an analogous function) responsive only to excursions of a particular polarity in the amplified signal. The particular embodiment described below is responsive to both positive and negative excursions of an amplified signal and is consequently less subject to the transients and distortion characteristic of half-cycle operation. Additionally, it was an object to provide this preferred embodiment with a well-defined detection threshold which would eliminate detection of low level signals falling in a prescribed magnitude range.